Variable speed drives chains and sprockets have been employed with bicycles for many years. The drawbacks of such systems are well known and are described in U.S. Pat. No. 4,030,373 to the inventor hereof. Therein is disclosed a variable ratio transmission for bicycles which includes a plurality of movable sheave segments, each sheave segment having a releasable, toothed retaining means which normally retains the sheave segment at a fixed radial position in a toothed track.
That structure is, essentially, a variable diameter pulley or sheave whose diameter is adapted to be selectively adjusted by the rider. A flexible belt is wrapped around and engages different adjacent sheave segments to impart rotary motion to the drive mechanism. The relative position of each sheave segment in its toothed track is adjusted only when a sheave segment comes out of contact with the drive belt.
The mechanism described in the '373 patent for locking each sheave segment into place after adjustment contains relatively small and highly stressed parts requiring close manufacturing tolerances. The setting mechanism is sensitive to both axial location and warpage. Locking surety also degrades somewhat with wear.
In U.S. Pat. No. 4,503,676 to the inventor hereof, a variable ratio drive mechanism is disclosed which also employs driving and driven sheaves, each of which is provided with a set of adjustable sheave segments. In that mechanism, individual sheave segments are one-piece, belt-loaded-locked units which engage saw-tooth shaped steps along associated trackways. The center line of each sheave segment is offset from a radial line so that the belt's force on each sheave segment applies an offset torque which forces the sheave segment's teeth into engagement with opposed saw tooth steps along one side of the track. When each sheave segment become free of the belt's force, it can be engaged by a shifter which causes its teeth to move out of engagement with the track's steps. The sheave segment is then radially movable in either an outward or inward manner. In order to unlock the sheave segment's teeth from engagement, means are provided to cause a modest amount of rotation of a segment's teeth so that they can ratchet up or down relative to the track's steps. This design is not suitable for small sheave diameters and for applications involving relatively resilient belts which are subjected to grossly fluctuating driving tensions. Furthermore, the design is adapted only to a single direction drive.
In U.S. patent application Ser. No. 140,232, filed Dec. 31, 1987 and entitled "Variable-Ratio Transmissions, Separately and In Bicycles" to the inventor hereof, there is disclosed still another transmission of the type that includes sheave segments coupled together by a drive belt. That transmission employs a sheave segment locking mechanism which runs the full length of each disk track in the drive mechanism. The locking mechanism described therein is controlled by a fixed path cam whose action is unrelated to the radial position of the sheave. More specifically, the locking mechanism is released and removed from interaction with an individual sheave segment by a cam means which is operative only when the sheave segment is out of contact with the drive belt. Under those circumstances, the sheave segment is free floating and can be either moved inwardly or outwardly by a shift mechanism. In this mechanism, positive and consistent lock-up is dependent upon light springs and free fitting, cooperating parts. Relatively close tolerances are required and lock-up surety decreases with wear.
Accordingly, it is an object of this invention to provide an improved variable ratio drive mechanism of simple design.
It is a further object of this invention to provide an improved variable ratio drive mechanism which exhibits substantial resistance to wear and positive lock-up.
It is a still further object of this invention to provide an improved variable ratio drive mechanism which is insensitive to belt resiliency and automatically compensates for wear.
It is another object of this invention to provide an improved variable ratio drive mechanism which is adapted to bidirectional operation and requires only relatively relaxed manufacturing tolerances.